i run a fairly large site... 20k members and growing...
i have found VB to be VERY slow, and requires much database optimaization of tables on a routine basis. it's my least favorite board system out there.
I have found phpbb to be completely unstable. It's crashed on my more than once. I won't go back to it after loosing 200,000 posts, and 15,000 member ID's and having to re-up the nearly 50MB of data.
wwwthreads is just to simple, as is Phorum. reminds me of 1996 BBS's with color.😃
and going back to the whole free thing- my vote goes to invisionboard.
Their templates/skins are hard to manipulate if you have safemode on, but other than that, a simple tar ball upload, extract, and bam- new theme. Their admin panel is great and super secure, requiring a new auth every time. They have support forums for both skins at ibskins.com. and a HUGE db of modifications at ibresource.com
20,000 members, 90,000 posts, 3 months being up on invision, and i have 0 complaints. The only bug i have found is that sometimes it gets greedy with server memory, and causes a php error due to fatal memory allocation. a simple refresh gets it going again though. Mind you, im also running an RC version, not the latest release, as I haven't had time to do the latest upgrade they released.